Mechanisms of uptake of ketone bodies by luminal-membrane vesicles

The energetics and location of renal transport of acetoacetate, beta-hydroxybutyrate, alpha-hydroxybutyrate and gamma-hydroxybutyrate by luminal-membrane vesicles from either whole cortex or pars convoluta or pars recta of rabbit proximal tubule were studied. Addition of either acetoacetate or beta-hydroxybutyrate or its analogues to dye-membrane-vesicle suspensions in the presence of Na+ gradient (extravesicular greater than intravesicular) resulted in absorbance changes indicative of depolarizing event(s). Valinomycin enhanced the Na+-dependent uptake of monocarboxylic acids, provided a K+ gradient (intravesicular greater than extravesicular) was present. By contrast, Na+-dependent uptake of these compounds was nearly abolished by ionophores that permit Na+ to pass through the luminal-membrane via another channel, either electrogenically (e.g. gramicidin D) or electroneutrally (e.g. nigericin). These results established that the Na+-dependent transport of ketone bodies and analogues by luminal-membrane vesicles is an electrogenic process. Eadie-Hofstee analysis of saturation kinetic data suggested the presence of multiple transport systems in vesicles from whole cortex for these compounds. Tubular localization of the transport systems was studied by the use of vesicles derived from pars convoluta and pars recta. In pars recta uptake of all these compounds was mediated by means of a single high affinity common transport system. Uptake of these compounds by vesicles from pars convoluta was carried out via a relatively low affinity but common transport system. The physiological importance of the transport systems is discussed.